By Monica Lindstrom | November 20, 2015 at 12:50 pmUPDATED: November 20, 2015 at 12:52 pm

(AP Photo)

The Supreme Court added a defense to Arizona’s DUI laws on Friday: A qualified card holder under the Arizona Medical Marijuana Act now has a leg to stand on when being prosecuted for a DUI.

What does this really mean? Let me break it down.

Arizona is very tough on DUIs and there are several kinds of DUIs that you can be convicted of.

The most common are DUIs where someone is impaired by alcohol to the “slightest degree,” when their BAC (blood alcohol content) is at or above .08, or when their BAC is at .15 or above, aka an extreme DUI.

There is also another type of DUI that is codified in A.R.S. 28-1381(A)(3). This statute says:

“It is unlawful for a person to drive or be in actual physical control of a vehicle in this state…while there is any drug defined in section 13-3401 or its metabolite in the person’s body.”

That meant if someone is driving and they have marijuana or its metabolite (what a drug breaks down into when our body processes it) in their body, then they are guilty of a DUI, regardless of whether they are impaired by that marijuana or not.

I say “meant” because, not too long ago, the courts changed this to say that a person cannot be convicted of a DUI under (A)(3) just because the “presence of a non-impairing metabolite that may reflect prior use of marijuana was in their body.”

But even with this change, courts were still not giving any consideration to qualified medical marijuana card holders. They did not have much of a defense, even if they were doing what they were permitted to do under a law passed by Arizona voters. They didn’t have the opportunity to tell the jury they had a medical marijuana card and that they were not impaired.

This issue made its way to the Arizona Supreme Court because it is an important issue that everyone — defendants, courts, prosecutors and defense attorneys — needs to learn to deal with. The issue of the Arizona Medical Marijuana Act interacting with Arizona’s DUI laws was decided Friday.

Defendants now have what is called an “affirmative defense,” meaning they are allowed to defend themselves based on the fact they are a qualified cardholder.

You have heard the sayings “innocent until proven guilty” and that it is the state’s burden to prove the defendant is guilty of every element of the crime “beyond a reasonable doubt.” Typically, there is never a burden on the defendant to prove his or her innocence.

An affirmative defense changes that a bit. The state must still prove guilt beyond a reasonable doubt but now, after that, the burden shifts to the defendant to prove their defense “by a preponderance of the evidence.” That preponderance of the evidence standard is much lower than the reasonable doubt standard, but it still places the burden on the defendant to prove they were not impaired.

So, according to today’s Arizona Supreme Court decision:

A qualifying patient under the AMMA can still be convicted of a DUI under 28-1381(A)(3) if the prosecutor proves, beyond a reasonable doubt, that the person was driving or was in “actual physical control” of a vehicle and had marijuana or its impairing metabolite in their body.

however;

A qualifying patient can now use an “affirmative defense” and prove by a preponderance of the evidence that the marijuana or the metabolite found in their body at the time of driving was in a concentration insufficient to cause impairment.

In other words, if cardholders can show they are qualified to use marijuana under the AMMA and they followed the rules and the amount of the substance in their bodies was not enough to impair driving, they can win their case.

This is a big decision for our state and finally puts the issue to rest. I find it likely other states will read this well-written and reasoned decision from our Supreme Court and use it to modify and change their DUI laws.

Look, bottom line: I am all for keeping DUI drivers off the street and for punishing them. But, just as importantly, I am for fairness. Even when I was a prosecutor, I thought it was unfair to convict someone for a DUI if he or she was not actually impaired and that was happening.

For a long time, if people smoked marijuana 10 days before they were driving, they could get a DUI because it was in their system, even if it wasn’t “impairing” them anymore. We were convicting people of driving while impaired when there was no proof they were impaired, only proof there was a drug in their system.

See the inherent unfairness?

Now, people who followed the law have a chance to prove they did so. Sounds like fairness to me.